The present invention relates to an apparatus and a method for practicing an image-forming process wherein a multilayer image is obtained on a photosensitive surface by an electrophotographic process and, more particularly, where such an apparatus and process prevents the splashing of toner during the exposure of the photosensitive surface at times subsequent to the first.
In the art of photo-reproduction a multi-color image is obtained by building-up multiple layers of toner; each layer of toner corresponding to a separate color. A photosensitive body, usually in the form of a drum, is electrified. The drum is then exposed to an image which disturbs its electrification in selected areas corresponding to the image. A toner, having an electric charge, is then applied to the surface of the drum and is repelled, due to its charge, from areas on the drum not corresponding to the image. This process may be repeated two or more times for two or more color toners to be applied. The individual layers of toners assume homopolarity by said electrification and are so repulsed by each other as to cause each to splash and to attach to and stain any part in the machine where there is a degree of Coulomb force in action caused by even the slighest potential difference. This defect causes a degredation of image quality through uneven exposure or fogging of images. The splashes are most conspicuous in reversal process as expalined below.
Exposure of an image is done by scanning of the photosensitive surface of the drum with a light image, having laser or LED as the light source, to form a static latent image on the drum which is developed by a toner comprised of chargeable particles. Since the blank area on the drum is much larger than the area covered with toner in an ordinary image, exposure of light to the blank area raises several problems. The life of the light source and the life of the image-forming body are shortened. Further, uneven scanning in the optical system produces lines in the blank area. For this reasons it is common practice to irradiate the colored part and not to apply light to the blank area when a latent image is formed. In this case, unlike the process in copying machines, a latent image is formed with a low potential in the colored area and a high potential in the blank area, thus the development is performed in a reversal process whereby the tone attaches to the low potential area of the latent image.
FIG. 1 illustrates, that when such a reverse development process is practiced, the inadequancy of the adhesiveness of the toner attaching to the image portion and how easily the toner splashes. FIG. 1(a) shows the condition following the first cycle of image exposure and development, wherein the development under a predetermined development bias causes the toner to attach to the image area where the electrical potential on the photosensitive body 1 is close to ground potential. FIG. 1(b)shows the condition when the second image exposeure is being carried out at a position close to the image area to which the toner is attached. A laser beam L, used for exposing the second image, has disturbed the electric field adjacent to the toner attaching to the image and caused the toner to become instable resulting in the removal or splashing of its particles.
FIG. 1(c) shows the condition of the exposure unit immediately after the secxond image exposure, wherein the laser beam, L, has caused part of the toner to splash and attach to a write unit 3 which is grounded and positioned facing an image unit 2.
Referring now to FIG. 2, there is now explained the cause of the toner splashing shown in FIG. 1.
In FIG. 2(a) through (e) a negative potential on the surface of photosensitive body is taken on the vertical axis and a position on the surface of a photosensitive body is taken on the horizontal axis. While the charge on the surface of the photosensitive body is shown in FIGS. 1 and 2 as being negative it should be understood that such a charge may also be positive.
FIG. 2(a) shows that the surface of a photosentsitive body has been charge uniformly to minus E Volts by means of an electrifier.
FIG. 2(b) illustrates a state wherein a photosensitive body has been exposed to light for an area I where the electric potential has approached close to 0 Volts.
FIG. 2(c) illustrates the state of development that, when the area I of exposure passes the developing device, a negatively charge toner is propelled from a developing sleeve. The toner has approximately the same potential as that of the uniformly charged potentail of the photosensitive body. It is propelled onto the area I of exposure on the photosensitive body and adheres thereto. The surface of the photosensitve body, where there is a toner image of the first color thus formed thereon, is uniformly charged agin for the second time by means of an electrifier. The first toner color image remains on the photosensitive body.
FIG. 2(d) shows the potential of the uniform charge for the second exposure cycle.
In the second cycle an image exposure takes place according to the image signals corresponding to the second color. FIG. 2(e) shows that the image is exposed to light at the area I'adhacent to the toner image of the first color. The potential at the area I' is lowered close to 0 volts. The potential at the area I' is higher than at the area I.
Each particle of the toner forming the toner image of the first color adheres to the surface of the photoreceptor because of the action of Coulomb forces. Electrically the toner particles, being charged with homopolarity, mutually repel each other. Since toner particles are negatively charged, a force to move the toner particles to a position higher in potential acts in the direction of the electric field formed there. If this force to move the toner particles is large, the toner jumps toward the area where the electric potential is large. FIG. 2(f) shows the state in which part of the toner attaching to the area I having a potential of --E Volts is jumping to move to the area I' whose potential is close to 0 Volts.
When a photoreceptor having a toner image thereon is exposed again for an image of a second color and toner jumps, part of the toner as it jumps impinges upon and attanches to the dust proof glass inside the image exposing device, lowering the transmissivity or causing unevenness of the exposure light as a result.
For the purpose of preventing toner from contaminating surfaces of the image forming apparatus, there have conventionally been provided an air stream directed into the apparatus to collect toner or direct it away, or use of a screen or brush, to remove umwanted toner particles or a sheet or a lid to shut off toner where necessary.
However, the application of an air stream to prevent umwanted toner particle attachment has raised problems in that various items of machinery, such as a fan, duct, filter are needed to collect loose toner particles. This makes the apparatus large, expensive and noisy. The use of a screen or brush has involved a problem that a screen can not be effectively employed in an exposure unit where toner splashes most. Similarly, slit or lid cut-offs are ineffective.